Digital cameras offer significant advantages over scanners for capturing document images and other images. Digital cameras are generally more portable than scanners, allowing users to image documents wherever they are and as the need arises rather than having to bring the document to a scanner. In addition, because scanners require that the document to be digitized be placed on a scanner platen, digital cameras are capable of capturing images of a wider variety of document types (e.g., larger documents or documents mounted in a frame) than scanners. However, the use of digital cameras creates difficulties that do not exist when using a scanner. For example, lighting conditions vary when using a digital camera, whereas the lighting conditions are generally controlled for scanners. In addition, use of a digital camera can introduce geometric distortions depending on various factors such as the angle and distance of the digital camera relative to the document, the lens used by the digital camera, and whether the document is situated on a flat or curved surface. Because scanners generally utilize a moving scanner head, at a fixed distance from a document which is held flat on the platen, these distortions do not typically occur in scanners. Another difficulty in capturing images of documents with a digital camera is the wide variety of different possible background shapes, patterns and colors.
Digital cameras (in particular, those included in mobile phones, smartphones, tablets, and other mobile devices) typically have a relatively small preview display as part of the device. This small screen makes it difficult for a user to assess the quality of the captured document by using the preview screen. Although the device display in many smartphones also acts as the preview or display screen for captured images, these screens may still be too small for a user to determine whether an image of a document is sufficient for the desired post-processing through visual inspection.
A typical application applied to scanned documents is using optical character recognition (OCR) to extract ASCII data for use later, such as indexing the document. Blurriness, noise, and uneven illumination will have a negative effect on the ability to OCR a document capture with a digital camera. Another problem is that flash use during image capture with a digital camera can cause a hotspot problem where part of the captured image data is obscured due to the flash. What is needed is a way to measure and analyze blurriness, noise, hotspot, and uneven illumination at the time of capture so the user can be alerted to re-capture the document with the digital camera.